1. Field of the Invention
The present invention generally relates to steam boiler furnaces and more specifically, to acoustic methods and apparatus for determining the existance of a fluid leak from pressurized tubing suspended within a furnace combustion space.
2. Description of the Prior Art
Pursuant to present-day paper pulp mill operations, raw wood is delignified by a thermo-chemical process comprising an approximately 350.degree. F. cook in the presence of sodium hydroxide, sodium carbonate, sodium sulfide and other sodium based compounds. Under such conditions, the lignin binder in the raw wood matrix which holds the natural cellulose fibers together reacts with the sodium compounds to form water soluble lignin-sodium complexes thereby permitting a water wash separation of the black, tar-like lignin from the fiber for manufacture of paper.
Although the sodium compounds used in the aforedescribed process are relatively inexpensive, the quantities consumed in the production of an average pulp mill necessitate an economical recovery and re-cycle of the chemical values. Moreover, such sodium-lignin complexes contain sufficient heat value and volatility to contribute favorably to the overall mill heat balance. These characteristics are combined in the liquor recovery furnace by fueling a boiler furnace with a concentrated flow stream of the spent or black pulping liquor. Combustion of the lignin fraction generates sufficient heat to evaporate the residual water vehicle and heat the steam required for the primary evaporative liquor concentration process. Residual ash, predominately sodium carbonate, falls to the furnace bed as a viscous smelt. Such smelt is cooled and dissolved in water to form the green liquor makeup stream from which the other fresh cooking liquor compounds are made.
Economics and thermal efficiency highly favor the use of such recovery furnaces but the practical, daily operation is potentially hazardous due to the risk of steam or water leaks from the heat transfer conduits therein. When the fluidized smelt in the recovery furnace bed is contaminated with water, a reaction of an explosive nature has been known to occur. Reference is given to U.S. Pat. Nos. 3,831,561 issued to T. Yamamoto et al. 3,615,175 issued to H. W. Nelson and 3,403,642 issued to A. Parkin for the scope of theory seeking to explain the mechanics of such explosive reactions. Regardless of the theoretical explanation, the physical fact remains that water contamination of a hot, recovery furnace smelt bed may destroy the furnace by a sudden pressure wave within the furnace combustion chamber. The most probable source of such water is from the boiler tubes, themselves. Over time and operation, the tubes develop leaks and water escaping from such leaks has no alternative but to fall into the smelt bed.
Traditional fuel (gas, oil and coal) fired steam generation furnaces have similar tubing leak problems but without the dire consequence of a smelt bed explosion. To detect the existence of such leaks in traditional fuel fired furnaces, acoustically sensitive systems have been developed to monitor combustion chamber sound emissions. These systems are designed to measure the magnitude of sonic energy from the combustion chamber atmosphere over a narrow frequency spectrum distinctive to fluid leaks from conduits. This is normally accomplished by means of a bandpass filter in the analog signal circuit of a sensitive microphone. Only leak distinctive frequencies are thereafter analyzed.
Unfortunately, the normal operating background noise of a furnace combustion chamber produces the same distinctive frequencies in considerable magnitude. It is the task of the acoustic leak detecting monitor, therefore, to distinguish a small, incremental increase in sonic energy due to a tubing leak from the large background energy normally present
In the case of traditional fuel fired furnaces, this electronic signal discriminating task is aided by the fact that combustion is sustained by a relatively steady flow of clean fuel. Solid combustion products (soot) are relatively small and the percentage of furnace operating time devoted to soot blower operation is relatively small.
Soot blowers are steam jets directed upon the external surfaces of the boiler tubing for the purpose of periodically dislodging accumulated soot and maintaining a high heat transfer rate through the tubing. In effect, therefore, soot blowers are deliberately and strategically applied steam leaks within the combustion chamber having a necessary maintenance purpose. When the soot blower operating cycle begins, signal reception from acoustic leak detectors must be either terminated or ignored for the purpose of tubing leak analysis. Since the soot blower operating cycle is relatively short and infrequent in traditional fuel fired furnaces, the signal interruption is of little consequence to an electronic monitoring apparatus reliant upon a continuous, steady-state signal to distinguish small percentage variations from a normal threshold magnitude.
In this respect, chemical recovery furnaces differ significantly from traditional fuel fired furnaces. The chemical recovery furnace fuel is "dirty" and of irregular volatility and fuel value. Soot production is enormous. Consequently, recovery boiler tube soot blowers must be used more frequently for longer operating intervals. Little time remains between soot blower operating cycles to accumulate and evaluate "normal" combustion chamber acoustics.
It is, therefore, an object of the present invention to provide an acoustic leak monitor for chemical recovery furnaces and other steam generating furnaces capable of reliably analyzing an extremely short duration acoustic source
Another object of the present invention is to provide a method and apparatus to digitally characterize an analog acoustic signal for immediate storage and subsequent retrieval and analysis.
Another object of the present invention is to provide an acoustic leak detection method and apparatus which converts microphone analog signals from an amplitude vs. time domain, as received, to a digital amplitude vs. frequency domain for comparative analysis.